The present invention relates to a method and a system for radio communication.
In cellular radio systems the number of cells used for covering a particular area should be as small as possible in order to minimize the costs. However, in order to maintain a sufficiently good signal to noise ratio (SNR) for a system having many mobile stations, the cell size should be as small as possible.
Thus, using a small cell size the same signals can be reused at other locations in the cellular radio system. In a system having M orthogonal signals or channels these are usually divided into K groups having the same or close to the same number of channels. In a hexagonal cell pattern with the base station located in the middle of the cell, see FIG. 1a, K can be any integer written as
K=(i+j)2xe2x88x92ij,
where i,j=0,1,2,3, . . .
Thus K can be K=3,4,7,9,12,13, . . .
A group of cells where each channel group is used once is usually termed cluster. K is therefore usually termed cluster size.
It is also common to use more advanced cell structures, where the base stations use directive antennas located in the corners of the cells, see FIG. 1b. A cell structure having a cluster size of 12 and 4 base station locations per cluster is denoted a 4/12 cluster and same way a cell structure having cluster size of 21 and 7 base station locations per cluster is denoted a 7/21 cluster etc. Such arrangements have the advantage that several base stations can be located at the same location, which is cost saving. The maximal distance from the base station to a mobile station will be increased but the number of interfering closely located base stations will be fewer.
In order to optimize the system for handling as many calls per cell as possible it is desired to have access to as many channels as possible per cell. This can be termed the capacity of the system (n), where
n=M/K
For a certain system the number of channels M which can be used is usually predetermined. Hence, in order to increase the capacity of the cellular radio system the number of groups K should be as small as possible. How small the number of groups K can be is determined by how sensitive to noise and interfering signals the signals are.
In order to increase the performance in transmission on a fading radio-channel, diversity is employed. Diversity means that the same signal is transmitted via two or more transmission paths. There are many different types of diversity arrangement such as: frequency diversity, time diversity, polarisation diversity and spatial diversity etc. In spatial diversity or antenna diversity, several antennas are used when receiving the signal. It can be shown that if the distance between two antennas is sufficiently long the signals at the different antennas will be uncorrelated.
The distance required is mainly dependent on the wavelength used in the transmission. A distance of about the wavelength divided by 2 can be sufficient to reduce the correlation to acceptable levels. The longer the distance between the antennas, the less correlation. However, very long distances may not be feasible. A normal value can be about 12-20 times the wavelength used.
Antenna diversity has the advantage of not requiring a larger frequency range. Also the implementation of an antenna diversity arrangement is quite simple.
In cities and other areas having a large concentration of mobile stations (mobile telephones), the frequency range available is very limited in comparison to the heavy concentration of mobile stations. In this environment frequency diversity systems and other types of diversity system increasing the required frequency range are not possible to use, since the number of channels available would be to low to cope with the large number of mobile stations. Therefore, the use of antenna diversity systems is preferred in many cases.
The receiver can use or weigh the signals from the different antennas in a number of different ways. The most common methods are:
Selection diversity: The receiver chooses the signal having the highest signal to noise ration (SNR) and uses this signal for the signal detection.
Maximum ratio combining diversity: The receiver uses the information from all the reception branches all at the same time. The signals from the different branches are given a weight which is proportional to the SNR at the respective branch. When using the method the individual signals must have the same phase when the addition takes place. Thus, it is necessary to implement a phase compensation circuit. The phase compensated and optimal weighted signal is then used as the output signal.
Equal gain combining diversity: This is a special case of the Maximum ratio combining diversity system. Thus, in order not to have to measure the momentary SNR in each reception branch the weight is set constant and equal for all the branches.
Switched diversity: This is a simple method. Thus, one branch at a time is used and switching between different branches is only carried out when the signal envelop drops below a certain threshold value.
The use of diversity systems is a powerful tool in order to avoid long fading conditions.
However, the diversity systems of today cannot be used for significantly increasing the channel capacity for cellular radio systems. The limitation in channel capacity is an increasing problem, since the frequency range available is limited and the number of mobile terminals is constantly increasing.
It is an object of the present invention to improve the channel capacity for cellular radio systems. This object is obtained by using a diversity arrangement in the base station, which makes use of the information obtained when receiving signals in an antenna diversity system for transmission purposes.
The radio transmission system is therefore equipped with means in the base station for extracting information regarding the current diversity conditions and the extracted information is used for controlling the transmission of the down-link signals according to a diversity (down-link) transmission scheme.
Thus, the same information used for the reception of the signal (up-link signal) is applied in a corresponding manner for transmission of the signal (down-link signal). This makes it possible to use less power in the transmitted signals whereby the cluster sizes can be reduced. This in turn results in a larger channel capacity according to the above.
The use of such a down-link diversity arrangement therefore provides a system operator with a tool by means of which he can easily increase the number of mobile stations without having to use a larger frequency range. For example it will be possible to reduce the co-channel interference so that a 4/12 cluster will have the same speech quality as for traditional base stations in a 7/21 cell cluster.